One of the known phenomena is a change in quality of an optical signal when being received at an optical receiver depending on a polarization state of the optical signal transmitted from an optical transmitter. This phenomenon is caused by, for example, Polarization-Mode Dispersion (PMD), Polarization Dependent Loss (PDL), Polarization Hole Burning (PHB), or Polarization dependent Gain (PDG).
This phenomenon may degrade the quality of an optical signal having a particular polarization state.
Considering the above, the optical transmitter described in Patent Literatures 1-4 performs polarization scrambling on an optical signal to be transmitted. The process of polarization scrambling modifies the polarization state of an optical signal each time a certain period elapses. The optical transmitter of Patent Literatures 1-4 pre-equalizes an optical signal to be transmitted. Pre-equalization is a technique of applying, to an optical signal to be transmitted in advance, a change of the reverse direction to a prospective change in the polarization state of an optical signal that would occur while the optical signal is being transmitted.
Patent Literature 1: United States Unexamined Patent Application Publication No. 2005/0226633
Patent Literature 2: WO 2011/096488
Patent Literature 3: Japanese Laid-Open Patent Publication No. 2010-109705
Patent Literature 4: United States Unexamined Patent Application Publication No. 2006/0127104